1. Technical Field
This invention relates to the technology of spray-forming bulk materials to create objects, and more particularly to making high-performance inserts without chemistry constraints using spray-forming techniques.
2. Discussion of the Prior Art
Inserts have been used to enhance the physical characteristics of certain parts of a component, particularly components in an automotive engine. For example, steel alloy valve seat inserts are used extensively in aluminum engine heads and in some high-performance or alternative fuel cast iron engine heads. The list of enhanced high-performance characteristics desired at the seat is often quite long, including increased ambient and high temperature wear resistance, higher creep resistance, higher thermal fatigue strength, (under repeated valve impact loading), better thermal conductivity, better corrosion resistance, lower manufacturing costs, and capability of being tribologically compatible with valve materials engaging the insert.
A common manufacturing approach that attempts to attain these characteristics is to make the inserts by powder metallurgy processes which involve several steps: weighing and blending of selected powder mixtures; compaction and green body formation in molds and dies; sintering and sometimes copper infiltration of the compact at respectively 1080.degree. C. and 1500.degree. C.; controlled cooling; post-sintering tempering heat treatment; and finally machining to the desired seat dimensions. This obviously is an involved process which adds considerable cost. To achieve the desired physical characteristics, chemical additions are made to the powder mixture of carbon, chromium, molybdenum (for wear resistance), cobalt and nickel (for heat resistance), and other additions to obtain better thermal conductivity or better self-lubrication. In ferrous based powder mixtures, the resulting product may have its matrix consist of pearlite, bainite or tempered martensite depending on the heat treatment used during compacting and sintering. The sintered insert will always have the same chemistry as the starting green compact with its micro-structure dependent on the heat treatment employed.
To obtain more optimum physical characteristics in inserts, very high concentration of certain additions (i.e. 15-25% wt. Cobalt; up to 20% wt. Pb) may be necessary, as well as the introduction of certain chemical ingredients, such as rare earths, which, unfortunately, inhibit or prevent sintering by powder metallurgy techniques. Moreover, powder metallurgy does not allow the introduction of low cost oxides or ceramics during processing; ceramics are very useful to achieve certain of the physical characteristics.
When an engine is run with alternate fuels such as natural gas or alcohol, powder metal valve seat inserts for internal combustion engine heads often are often inadequate. Powder metal valve seat inserts, when used for intake valve seats and alternate-fuel engines, often contain too little self-lubricant, such as lead, and thus prematurely wear severely. Lead is also undesirable as an embedded self-lubricant since it can foul catalytic surfaces used in treating emissions.
The prior art has not attempted to use thermal spray-forming techniques to make high-performance inserts. The Osprey spraying technique uses a refractory tundish to supply a stream of molten metal which is atomized under inert atmosphere or vacuum to spray-form bulk materials; however, difficult and exacting procedures are necessary to control the molten bath and stream, thus limiting its use to making inserts economically.